Redesigned the InGauge USB version 2.1 circuit to pass CE/FCC tests under NSTDA’s ITAP project

How the brief came about

The brief in this case was to make every InGauge USB unit delivered to customers work reliably and pass CE/FCC testing. InGauge USB is a device that takes values from digital measuring instruments, such as digital vernier calipers, and sends them to a computer or tablet over a USB cable as an HID (Human Interface Device — a device the computer sees as a keyboard). Measured values can therefore go straight into a field in a program without being written down on paper.

The first version (V1.0) had to be soldered and wired on the circuit board unit by unit. Some units could not read values from digital vernier calipers, or misread the signal from the foot switch. Units assembled incorrectly had to be scrapped, and the circuit had not been designed to support CE/FCC standards, which we aimed to pass before delivering the device to customers.

Requirements and constraints

The new version had to perform all the original functions of InGauge USB and fix the weaknesses of the first version within the project timeframe. The main requirements and constraints are as follows.

• Receive values from digital measuring instruments and send them to a computer via USB HID, as if typed on a keyboard
• Support a foot switch via an XLR connector so that staff can send values by stepping on it
• Design the circuit to support CE and FCC testing
• The prototype must be tested under high temperature and humidity before production
• Reduce work that requires skilled staff, because the first version had to be soldered and wired unit by unit
• Complete within the 6-month project period (20 Jul 2020 – 20 Jan 2021)

Why we redesigned the circuit instead of fixing existing units one by one

We redesigned the circuit because analysis of the earlier version found that the errors lay in the production method. Soldering and wiring each unit required skilled staff, and connecting many wires made mistakes easy. A printed circuit board (PCB) that arrives fully assembled from the manufacturer therefore removes manual soldering and wiring from our process.

We did not change what the device does; we changed the circuit, the production method and the test method. Protection circuits against static electricity, reverse current and short circuits were included from the design stage so that the circuit would support CE/FCC testing. Our team wrote new control firmware following a State Diagram (a diagram of operating states).

Responsibilities on this work were clearly divided. The project's consulting expert handled circuit analysis and design, while our team handled the firmware, the enclosure and the production steps.

Deliverables

When the project closed, we had InGauge USB version 2.1 along with production and testing procedures for producing this version. The main items are as follows.

• New circuit and PCB that arrive fully assembled from the manufacturer, with protection circuits against static electricity, reverse current and short circuits
• Aluminium enclosure that we designed and had manufactured by a contractor
• Control firmware based on a State Diagram, already tested at a real site
• Pre-assembly incoming inspection procedure for the circuit, with test results recorded as digital files
• Cable labels, and a manual with example clips for each step
• CE and FCC test results from an external laboratory

How InGauge USB version 2.1 developed from a project into a product

We developed version 2.1 in sequence, from finding the cause, designing the circuit and testing the prototype to setting up production steps and sending it for CE/FCC testing. After the project closed, this tested version went on to become the InGauge product that we sell and rent out.

1. 1

   Find the cause of the problems in the earlier version

   Looked for the cause in the earlier version's production method.

2. 2

   Redesign the circuit and PCB

   Redesigned the circuit and PCB based on the causes found.

3. 3

   Test the prototype and make the aluminium enclosure

   Tested the prototype under high temperature and humidity, then made the enclosure.

4. 4

   Develop firmware and set up incoming inspection

   Wrote the firmware, tested it at a real site and set up the incoming inspection procedure.

5. 5

   Send for CE and FCC testing

   Sent units to an external laboratory for CE/FCC testing.

For real-world use after the project, see the case on renting and selling InGauge USB to factory QC departments, which uses InGauge USB version 2.1

Results confirmed by the project summary report

According to the project summary report, InGauge USB version 2.1 passed CE and FCC testing and takes 15–25 minutes per unit to assemble and test, compared with 75–90 minutes for V1.0. The work takes 1 staff member instead of 2, and units passing testing rose from about 75 percent to more than 99 percent.

The 15–25 minutes covers only assembly and testing at our company, because the circuit boards arrive fully assembled from the manufacturer. The figures in this section are values that we recorded and reported to the project ourselves.

WALTEK Testing Group in Shenzhen tested version 2.1 to CE (EN 55032 / EN 55035) and FCC Part 15 Subpart B standards. The report also confirms that this version works with digital measuring instruments from 2 brands, Mitutoyo and Teclock.

What we learned

This work offers principles for developing electronic devices that have to be produced repeatedly across many units.

• Fix the production method before adding inspection. If errors come from manual soldering and wiring, designing the circuit board to arrive fully assembled from the manufacturer removes that point of failure from the process.
• The standards to be passed are design requirements, not a final step. The first version was not designed to support CE/FCC, so in version 2.1 the protection circuits against static electricity, reverse current and short circuits were in the design from the start.
• Production steps must be transferable. Incoming inspection with results recorded as digital files, cable labels and example clips in the manual mean that quality does not depend on only a few skilled people.

How this approach can apply to your work

If you are developing an electronic device for repeated production, you can follow the same work sequence as this case: find the cause in the production method, design the circuit to support the standards it must pass, then set up an incoming inspection procedure that can record results. What you need to adapt is the standards and the operating conditions of your own device.

If you plan to use InGauge on your production line

If you plan to buy or rent InGauge rather than develop your own device, check these points before deciding

• The brand and model of the measuring instruments you use. The project report covers testing with Mitutoyo and Teclock; other sub-models should be tried in real use first
• The destination program that will receive the values, such as Excel, an existing QC system or the InQC app on a tablet, because InGauge sends values as if typed on a keyboard
• The nature of your measuring work, whether a foot switch is needed, and the number of measuring points that need a device connected
• Outright purchase or rental. InGauge is available both to buy outright and to rent monthly. See prices on the product page